In our earlier applications including Ser. No. 13/599,828, filed Aug. 30, 2012, Ser. No. 13/838,806, filed Mar. 15, 2013, Ser. No. 13/939,965, filed Jul. 11, 2013, Ser. No. 14/197,596, filed Mar. 5, 2014, and Ser. No. 61/948,212, filed Mar. 5, 2014, we disclose self-suspending proppants which take the form of a proppant particle substrate carrying a coating of a hydrogel-forming polymer. As further described there, these proppants are formulated in such a way that they rapidly swell when contacted with aqueous fracturing fluids to form hydrogel coatings which are large enough to significantly increase the buoyancy of these proppants during their transport downhole yet durable enough to remain largely intact until they reach their ultimate use locations. The disclosures of all of these earlier applications are incorporated herein by reference in their entireties.
Preferably, these self-suspending proppants are also free-flowing when dry. In this context, “dry” will be understood to mean that these proppants have not been combined with a carrier liquid such as would occur if they were present in an a fracturing fluid or other suspension or slurry. In addition, “free-flowing” will be understood to mean that any clumping or agglomeration that might occur when these proppants are stored for more than a few days can be broken up by gentle agitation.
It is well known that calcium and other divalent ions can substantially retard the ability of anionic hydrogel-forming polymers to swell when contacted with water. In this context, an “anionic hydrogel-forming polymer” will be understood to mean a hydrogel forming polymer whose hydrogel-forming properties are primarily due to pendant carboxylic groups but may also be due to other anionic groups such as sulfonate, phosphonate, sulfate and phosphate groups. This problem can be particularly troublesome when such polymers are used in hydraulic fracturing applications, because the source water used to make up the fracturing fluids used for this purpose often contain significant quantities of these ions. To this end, the self-suspending proppants of our earlier disclosures can also be adversely affected by these ions, as reflected by a reduction in the degree to which these proppants swell and hence the degree to which they become self-suspending when contacted with their aqueous fracturing fluids